Composite structures comprising fibers in a matrix material have received increased attention in recent years for a wide variety of applications including commercial, civil, industrial, military, and other applications. For example, carbon fiber reinforced polymer (CFRP) materials are increasingly being used for structural applications, such as rehabilitation and upgrade of concrete structures. The invention encompasses testing of any materials comprising aligned elements such as fibers in a dielectric matrix, and is described herein with emphasis on CFRP materials for illustration purposes only.
Carbon fiber reinforced polymer materials have unidirectional carbon fibers impregnated with epoxy. Such composites are externally bonded to concrete members to provide supplemental flexural, shear, or confining reinforcement. Proper adhesion of the CFRP material to the concrete structure can be critical to achieving the desired transfer of stresses to the CFRP material. However, disbond between the CFRP material and the concrete can occur due to, for example, improper application, moisture, or impact damage. Similarly, delamination within the CFRP material can interfere with internal distribution of stresses and overall performance.
Damage to concrete (for instance, impact damage) under CFRP laminates can be also critical to providing the engineering properties for which CFRP laminates are being employed because the damages of concrete under CFRP laminates are not visible through the CFRP.
Heretofore, nondestructive testing techniques have not always been effective for testing composites for conditions such as disbanding, delamination, and damage to concrete under CFRP. Microwave techniques have been employed whereby microwaves are directed at a specimen, and reflected microwaves signals are evaluated for changes indicative of anomalies in the specimen which may be indicative of defects. Standoff distance—distance between the specimen surface and the signal sensor—often changes while scanning a specimen due to local relative tilt of the specimen surface, shaking or movement of the specimen or the probe, specimen roughness, bulging, or other reasons. These changes can significantly influence the properties of the reflected microwave signal. Subtle anomalies such as disbonds and delaminations can be easily masked by standoff distance change influences.
U.S. Pat. No. 6,359,446 discloses nondestructive testing of dielectric materials using monochromatic, phase coherent electromagnetic radiation, preferably in the 5 to 50 GHz range (microwave range). A portion of the impinged beam is combined with the signal reflected by the test specimen. The signals combine to produce an interference pattern. The measurements are sensitive to variations in standoff distance.
One method which has been attempted to address the standoff distance issue has been the use of a mechanical system such as rollers that maintain the standoff distance fairly constant during the scan. However, this method is ineffective when the specimen has local surface roughness/bulging that may be smaller in spatial extent than the inspection area of the open-ended probe. Moreover, this requires contact between a component of the test device and the specimen.
Another attempted method involves measuring standoff distance variation during the scan and then subsequently processing the data to subtract the effect of standoff distance variation from the reflected signal. In particular, U.S. Pat. No. 6,462,561 discloses a near-field sensor including circuitry which compensates for variations in standoff distance. The method by which standoff distance is removed as an influence, however, requires that there be contact between a component of the test device and the specimen; so the method is not “non-contact.” In particular, it employs a potentiometer distinct from the waveguide probe. This can render the technique inoperable for certain applications where access to the object surface is limited by the placement of the object or its shape. Also, its accuracy is limited by the fact that the potentiometer and waveguide probe, being distinct components, are not taking measurements simultaneously from the same location.
In another method a differential sensing system is used to subtract out of the vibrational signals or noise as disclosed in an alternative aspect of U.S. Pat. No. 5,886,534. Instead of a single antenna and reflector arrangement in the original millimeter wave system 10 for on-line inspection of thin sheet dielectrics, a dual arrangement of a pair cross polarized antennas 34A and 34B and corresponding dual reflectors 16′ is provided in the alternative, differential sensing system 10′ as shown in FIG. 6 of this patent. The separation between the two sensing locations is provided such that there is no coupling or interference between two signals of the antennas. Because of close proximity, for example, about 3 inches, vibrational signal levels are expected to be identical at two locations. Consequently, if a difference of the two sensor signals is obtained the vibrational signals or noise are subtracted by a differencing scheme consists of a millimeter wave hybrid coupler 92 and a differential detector 96. If, on other hand, oriented defects such as warp or picks defects lay in the field of view of both antennas, the deferential sensing would give a defect-related signal because one sensor will be more sensitive that the other, depending on the defect orientation. This method and system provides inspection of thin sheet dielectrics with removal of influence of standoff distance variation caused by the vibration of the sheet. However, this method is ineffective when the specimen has surface roughness/bulging that may be smaller in spatial extent than the separation between the two sensing locations (about 3 inches) or when defects are not oriented. Moreover, this method and system provides only simple subtraction of two cross-polarized reflected signals that can be useful in limited applications because interaction of the cross-polarized signals with the specimen can be different.